The present invention relates to dental implants of the type in which a base is initially implanted in a patient's jaw where it is left for a period of time to permit bony ingrowth into the surface of the base to permanently affix it in the bone so that the base effectively forms a unit with the surrounding bone. Thereafter, a spacer; for example, made of titanium, is secured to the base and a dental prosthesis; for example, in the form of a crown or a bridge, is anchored to the base. A so-called cup, which may be an integral part of a bridge, for example, rests on top of the spacer and is secured with a retaining screw or nut.
While space is severely limited, dental prostheses are subjected to large forces acting in different directions which can put severe strain on the implant structure. The problem is particularly severe because in a space smaller than the cross-section of the tooth a tubular spacer must be located which transfers the forces to which the implant is subjected to the base implanted in the bone. The spacer must be configured so that there is room for anchoring the prosthesis with a screw and/or nut.
While screw and/or nut connections are very useful to assemble and install prostheses, any relative rotation thereof can lead to a highly undesirable loosening of the implant, which, if permitted to occur, requires under the best of circumstances an immediate visit to a dentist, but which can also lead to damage or the destruction of the implant and/or surrounding body tissue.
Some implants of this type employ a spacer screw which is concentrically disposed inside the tubular spacer and which has one end threaded into the base anchored in the bone. The other end of the spacer screw has a threaded hole which receives a retaining screw that engages the earlier-mentioned cup and presses it against the tubular spacer and hence the implanted base to secure the cup and therewith the dental prosthesis to the base.
Since the implant components, and in particular the base, the spacer screw, the cup and the attachment screw or bolt, are stacked, the thread sizes decrease over the height of the stack so that the diameter of the retaining screw thread can be no more than about two-thirds the diameter of the spacer screw thread engaging the base. Moreover, the retaining screw head is similarly small so that frictional forces which hold the retaining screw in place after it has been tightened are relatively small. The large, multidirectional forces to which the implant is subjected can lead to a loosening of the retaining screw, which in turn results in a highly undesirable and potentially damaging loosening of the dental prosthesis.
U.S. Pat. No. 4,746,293 discloses an improved variant of such a dental implant in which, instead of a separate retaining screw, the spacer screw includes a threaded stud at its free end which projects into the cup and which is engaged by a nut which, upon tightening, presses the cup against the spacer and the implanted base to thereby secure the dental prosthesis to the base. In the arrangement disclosed in that patent, the threaded stud on the spacer screw can have a relatively larger diameter than the earlier-discussed threaded holes, which enhances friction between the stud and the nut and thereby makes a loosening of the nut less likely.
However, in the arrangement disclosed in that patent, space for the nut is limited and it engages the cup along a narrow annular bearing surface the outermost diameter of which is less than an inner diameter of the tubular spacer. This narrow annular bearing surface contributes little to frictionally securing the nut against rotation. Further, the dental prosthesis disclosed in that patent is constructed to permit limited relative pivotal movements between the cup and the tubular spacer. Giving the nut an outer diameter less than the inner diameter of the tubular spacer and interposing an O-ring between them facilitates such relative pivotal motions.
A drawback of this arrangement is that the axial holding forces generated by the nut cannot be transferred to the tubular spacer in an axial direction, which can lead to the generation of undesirable force components between the cup and the spacer. Another drawback of this arrangement is that the frictional forces generated between the components of the implant are relatively small and, therefore, do not efficiently prevent turning of the nut and the resulting loosening of the implant.
Accordingly, there is presently a need for a dental implant which is effective in securing the dental prosthesis to the implanted base and prevents relative rotation of the implant components, and particularly of the retaining nut or screw and a resulting loosening of the prosthesis.